1. Field of the Invention
The present invention relates to a tire slip control device for a vehicle such as a loading vehicle. More particularly, the present invention relates to a tire slip control device for controlling slip of tires occurring when a torque converter stalls (i.e., when an output side of the torque converter is restricted and a slip ratio of the torque converter is thereby increased to 100% with the maximum torque being transmitted) or is in an almost-stall condition.
2. Description of the Related Art
For example, as shown in FIG. 5, a wheel loader 51 for loading operation is generally adapted to increase an engine speed (revolution speed of an engine E) to its maximum when pushing a bucket 52 into earth and sand or rocks to excavate and scoop up the same. At this time, a torque converter TC operates in a stall condition. However, driving wheels W tend to slip if a driving force is higher than a static frictional force. Such slip causes tires to wear in a short time. Also, slip on rubble causes the tires to be damaged. In particular, in case of a construction machine such as the wheel loader, the wear of the tires brings about an increase in maintenance cost. In FIG. 5, reference numeral TM denotes a transmission.
Conventionally, when such slip occurs, this is detected and the driving force to be transmitted to the wheels is restricted to suppress the slip. The prior arts are disclosed in Japanese Patent Application Publication No. Hei. 4-2560, Japanese Patent Application Publication No. Hei. 6-58345, Japanese Patent Application Publication No. Hei. 7-109240, and Japanese Patent No. 2528456.
In the conventional tire slip judgment method according to these prior arts, it is judged that slip has occurred by detecting difference of a revolution speed between right and left driving wheels, by detecting difference of acceleration between the right and left driving wheels, or by detecting difference of a revolution speed between driving wheels and driven wheels.
When the occurrence of slip is confirmed, a rotational driving force of the driving wheels is controlled to suppress the slip. Specifically, in order to suppress the slip, the rotational driving force of the driving wheels is suppressed by braking, the rotational driving force of the driving wheels is suppressed by incompletely engaging a clutch, and the rotational driving force of the driving force is suppressed by reducing fuel supply to the engine to thereby reduce output torque and rotation of the engine.
However, when it is judged that the slip has occurred by detecting the difference of the revolution speed between the right and left driving wheels to thereby detect the slip, it is necessary to provide rotation detectors on right and left axles of the driving wheels, respectively. In addition, when there is an error due to the difference of the revolution speed between inner and outer wheels that is generated when a car body is turning or both of the driving wheels have slipped, it is impossible to judge whether or not the slip has occurred. The same problems occur when detecting the difference of acceleration between the right and left driving wheels. Furthermore, when detecting the difference of the revolution speed between the driving wheels and the driven wheels, it is necessary to additionally provide the rotation detectors on the driving wheels and driven wheels. Consequently, the conventional slip judgment method based on the difference of the revolution speed between the driving wheels and the driven wheels is not applicable to four-wheel-drive vehicles such as the wheel loader.